A so-called twist-type cap is sometimes used for opening and closing the mouth of a flat container body (e.g. Patent Documents 1 to 3).
A twist-type cap can be attached to the mouth of a container body simply by pressing axially, and can be removed simply by rotating about 45°, for example, in an opening direction to open the mouth. Specifically, locking claws to elastically lock into a locking portion on the outer perimeter of the mouth are extended down inside the cap. Thus, by axially pressing the cap, locking the locking claws into the locking portion on the mouth side, the cap can be attached to the mouth in one action to close the mouth. When the cap is rotated by a predetermined angle, a linear portion at its lower edge goes up on an oblique shoulder portion of a flat container body, and the cap is driven in a removing direction and can be easily removed.